The invention relates to an arrangement for communicating messages via a low-voltage electricity supply system in accordance with the preamble of Claim 1, and also to an adapter for use in the said arrangement.
It is known for messages such as telephone calls, faxes, digital data, etc., also to be transmitted via low-voltage electricity supply systems. For this purpose, the messages are coded and impressed on a high-frequency carrier signal in a known manner, e.g. by frequency modulation. The modulated carrier signal is coupled into the low-voltage electricity supply system by the respective transmitter and is coupled out again in the same way by the respective receiver and decoded. Message transmission systems which use an electricity supply system in this way are described e.g. in WO-A-95/19 537.
However, high-frequency interference can arise in low-voltage electricity supply systems as a result of various processes, which interference is superposed on the carrier signal, is concomitantly coupled out and, under certain circumstances, appreciably impairs the quality of the message transmission. This makes it necessary to repeat communications, which entails a reduction in the data rate. Suppressing such interference at the receiver is often possible only with a considerable outlay on circuitry, or is not possible at all.
The strongest interference signals often originate from the local proximity of apparatuses participating in the message exchange. In particular, all kinds of apparatuses connected to the mains often generate interference signals with a pronounced high-frequency component e.g. during switching processes, which signals, if they pass into the mains in the vicinity of an apparatus connected for reception, reach the said mains with almost no attenuation and appreciably interfere with the message reception.
In addition, reflections often occur at open line terminations. It frequently turns out that the length of a cable between plug and apparatus constitutes precisely a quarter of the wavelength of the carrier signal or an odd-numbered multiple thereof. This leads, irrespective of whether or not the apparatus is switched on, to particularly pronounced reflections which are superposed on the carrier signal. Many instances of interference which greatly impair the quality of the message transmission pass into the electricity supply system in this way, especially in office buildings containing numerous connected apparatuses.
In order to protect connected apparatuses, mains filters integrated in mains plugs are known, with inductances in phase conductor and neutral conductor and capacitive coupling of the same to the earth conductor connected to a cable screen (see e.g. U.S. Pat. No. 4,614,924), the capacitances considerably increasing the volume of the plug, in particular its length and also the costs. U.S. Pat. No. 5,266,055 also discloses a three-pole mains plug in which only inductances are used. In both cases, the inductances serve only for protecting the connected apparatus against high-frequency interference signals. For this reason, the plug is also fixedly connected to the apparatus in each case. The last-mentioned document also shows connecting parts which serve for suppressing interference within an apparatus. Both in the case of the mains plug and in the case of the connecting part, a short conductorxe2x80x94a straight conductor piece or a half-loopxe2x80x94is embedded in ferrite material in each case. However, only small inductances can be realized in this way, and they only suppress signals of very high frequency.
The invention is based on the object of specifying an arrangement of the generic type which is significantly improved compared with known arrangements of the generic type in respect of its suitability for transmitting messages, and also an adapter suitable for use in the arrangement according to the invention.
By virtue of the invention""s arrangement for communicating messages via the low-voltage electricity supply system, the latter is better adjusted to transmitting messages. Interference of the aforementioned type is intercepted or suppressed at source and kept away from the mains. Thus, in particular, the reception of messages is not impaired by virtually unattenuated interference from the local proximity. This applies particularly when the frequencies utilized for the data communication are comparatively high. The interfering effect of cable sections situated between plugs and apparatuses is also suppressed and a line termination which is favourable for the transmission response of the low-voltage electricity supply system is produced in each case.
The suppression of interference also means that more capacity is available for the communication of messages, since the total permissible interference levelxe2x80x94which must also include the carrier signalxe2x80x94is usually limited by legal provisions. This favourable effect is additionally enhanced by the fact that in the case of a low interference level, a weaker carrier signal also suffices.
The adapter according to the invention affords a particularly simple and highly cost-effective possibility for realizing the arrangement according to the invention. It can be constructed very compactly, in particular it can be short, and can be fitted easily and rapidly as required and can likewise easily be removed again as well, with the result that the electricity supply system can largely be kept free of interference. The use of inductor coils saves space and avoids saturation effects.